Impact of a low level of CO2 enrichment on soil carbon and nitrogen pools and mineralization rates over ten years in a seasonally dry, grazed pasture

We have followed the responses of the properties of a sandy soil (0–50 mm), including carbon (C) and nitrogen (N) concentrations, to 10 years of ambient and elevated (475 μl l−1) CO2 in a FACE (Free Air CO2 Enrichment) experiment in a grazed pasture in North Island, New Zealand. We have previously reported results for the first 5 years where, at this relatively low level of CO2 enrichment, most soil properties changed gradually and non-significantly. Here we add data from a further 5 years of enrichment and find that most soil properties had changed significantly. Soil moisture was greater under elevated CO2 but we could find no evidence that this was the cause of changes in other properties, suggesting that increased inputs of C and N in the high CO2 regime were driving soil differences. When soil total C and N were adjusted to account for differences in starting concentrations there was evidence of increased pool sizes and increased C/N under elevated CO2. Microbial C and N pools also increased with CO2 as did heterotrophic respiration and labile C and N fractions. There were few changes in N mineralization but no evidence for reduced rates under elevated CO2. There was little evidence for a direct effect of changes in soil properties on plant biomass responses to CO2 but the changes we observed relate to pools and fluxes that are known to be important in both production and environmental outcomes in grassland systems.